Powdered carbon as a water filtration media has come into extensive, relatively recent use for supplanting other types of media, principally cellulose. While neither cellulose nor powdered carbon are inherently bacteriostatic, the carbon does have the advantage of permanence, i.e., is non-biodegradable, and has good affinity for organic pollutants. The organics, due to their ever-growing quantity, chemical diversity, unknown long-range effect, and potential for highly serious health hazards, have been the subject of increased regulatory attention. Accordingly, water filters containing powdered carbon have posed a good approach to improved water quality.
A particularly useful filter media and configuration is shown in the Oliver U.S. Pat. No. 3,055,503 which employs a pleated filter element in a quick-change type of cartridge. The drainage grid has a balanced pressure design which permits even deposition of the precoatable carbon filter particles thereon.
A major problem with all types of water filters is that bacteria adsorb to the solid/liquid interface, and accordingly can multiply in and on the filtration media. If there is no bacterial control, the effluent bacterial count can rise beyond acceptable limits, particularly during times the filter is not in use.
As noted, carbon is not inherently bacteriostatic. One of the better, well-known bacteriostatic agents is silver. For example, U.S. Pat. No. 2,040,806 shows a fibrous material treated with silver has disinfectant properties. U.S. Pat. No. 2,701,792 shows that agents having bacteriostatic properties can be used to treat activated carbon filtering media. U.S. Pat. No. 2,785,106 treats alpha cellulose with silver nitrate.
However, silver applied to granular or powdered carbon for filter media poses several hitherto unsolved problems. Principal among these is that the process of treatment is extremely messy and in many instances difficult to control the quantity of silver deposited on the powdered carbon. Further, the price of silver is increasing, and the process has become extremely costly. Also, the energy-extensive processing costs of treating all of the filter media, as composed to treating only a small portion of it, is best avoided. Even more significant, the treatment of the powdered carbon with the silver can change its properties so that it is no longer efficiently suitable for filtration, particularly in the case of the precoat types of filter assemblies. Treatment of the powdered carbon in such filters typically requires increase in the amount of carbon due to the loss of sorbative properties, which in turn results in increased costs.
All types of fibers are not a good filtration media in view of the fact that they may be difficult to handle when wet, may not precoat well as they may tend to become lumpy, may not disperse well, and may be biodegradable. They have a lower affinity for organics than carbon. Thus, a filter of fibers alone, even where treated with silver, would require large quantities in order to overcome the inherent problems of low filtering efficiency. We do not believe that cellulose used alone would be suitable in a precoat-type filter because of its characteristics. Further, due to the biodegradable nature of untreated cellulose, we would expect erratic performance due to potentially varying rates of bacterial degradation of the cellulose from filter to filter.
Accordingly, there is a very significant need for an effective filtration media which has the advantages of precoatable powdered carbon, yet can have the advantages of bacteriostatic properties to prevent bacterial build-up when that media is used in various types of filters, and is capable of passing the current EPA standards in regard to effluent bacterial count, filtration efficiency, and low silver elution.